1. Field of the Invention
This invention relates generally to electrochemical fuel cell stacks. More specifically, the present invention relates to electrochemical fuel cell stacks comprising a plurality of fuel cell assemblies having integrated diodes.
2. Description of the Related Art
Fuel cell systems are currently being developed for use as power supplies in a wide variety of applications, such as automobiles and stationary power plants. Such systems offer the promise of delivering power economically while providing environmental and other benefits. To be commercially viable, however, fuel cell systems should exhibit adequate reliability in operation, even when the fuel cells are subjected to conditions outside their preferred operating ranges.
Fuel cells convert reactants, namely, fuel and oxidant, to generate electric power and reaction products. Fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. A catalyst typically induces the desired electrochemical reactions at the electrodes. Fuel cell types include alkaline fuel cells and solid polymer electrolyte (SPE) fuel cells, which comprise a solid polymer electrolyte and operate at relatively low temperatures.
SPE fuel cells employ a membrane electrode assembly (MEA), which comprises the solid polymer electrolyte or ion-exchange membrane disposed between the two electrodes. Each electrode contains a catalyst layer, comprising an appropriate catalyst for facilitating the desired electrochemical reaction of the fuel and oxidant, located adjacent to the solid polymer electrolyte membrane. The catalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support). The catalyst layers may contain an ionomer similar to that used for the solid polymer electrolyte membrane (e.g., Nafion®). The electrodes may also contain a porous, electrically conductive substrate that may be employed for purposes of mechanical support, electrical conduction, and/or reactant distribution, thus serving as a fluid diffusion layer. Separator or flow field plates, for directing the reactants across one surface of each electrode or electrode substrate, are disposed on each side of the MEA.
A broad range of reactants can be used in SPE fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a SPE fuel cell, fuel is electrochemically oxidized at the anode catalyst, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the electrolyte, to electrochemically react with the oxidant at the cathode catalyst. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant at the cathode catalyst to generate water reaction product.
In operation, the output voltage of an individual fuel cell under load is generally below one volt. Therefore, in order to provide greater output voltage, multiple cells are usually stacked together and are connected in series to create a higher voltage fuel cell stack. End plate assemblies are placed at each end of the stack to hold the stack together and to compress the stack components together, thereby sealing and providing adequate electrical contact between various stack components. Fuel cell stacks can be further connected in series and/or parallel combinations to form larger arrays for delivering higher voltages and/or currents.
Electrochemical fuel cells are occasionally subjected to a voltage reversal condition, which is a situation in which the cells are forced to the opposite polarity. Opposite polarity may be deliberately induced, as in the case of certain electrochemical devices known as regenerative fuel cells. However, power-producing electrochemical fuel cells connected in series are potentially subject to unwanted voltage reversals, as is the case when one of the cells is forced to the opposite polarity by the other cells in the series. In fuel cell stacks, this can occur when a cell is unable to produce, from the normal electrochemical reactions occurring within it, the current being produced by the remainder of the cells in the stack and being directed through the affected cell by virtue of its being in series with the remainder of the cells. In addition, groups of cells within a stack can undergo voltage reversal and even entire stacks can be driven into voltage reversal by other stacks in an array.
Aside from the loss of power associated with one or more cells going into voltage reversal, this situation poses durability and reliability concerns as well. Undesirable electrochemical reactions may occur, which may detrimentally affect, or degrade, fuel cell components. For example, when there is an inadequate supply of fuel (e.g., fuel starvation due to water flooding at the anode, fuel supply problems, and the like) to a SPE fuel cell, there can be a rise in the absolute potential of the fuel cell anode leading to the electrolysis of water present at the anode and oxidation (e.g., corrosion) of the anode components. Such component degradation reduces the reliability and performance of the affected fuel cell, and in turn, its associated stack and array. Sufficiently high voltages on the anode can also lead to electrical shorting in the cell. The membrane electrolyte with its relatively high resistance is heated when such a high voltage is applied across the cell. With sufficient heating, the membrane melts resulting in the formation of holes and shorting of the cell. As disclosed in International Publication Nos. WO 01/15247 and WO 01/15249, a SPE fuel cell can be made more tolerant to voltage reversal by incorporating an additional catalyst at the anode to promote the electrolysis of water and/or by enhancing the presence of water at the anode through modifications to the anode structure. In these ways, more of the current forced through the cell may be consumed by the electrolysis of water than by the oxidation of the anode components.
In addition, adverse effects of voltage reversal may be prevented, for instance, by connecting diodes, capable of carrying the stack current, across each individual fuel cell (as disclosed in International Publication No. WO 00/49673) or by monitoring the voltage of each individual fuel cell and shutting down an affected stack if a low cell voltage is detected. However, since fuel cell stacks typically employ numerous fuel cells, such approaches can be quite complex and expensive to implement.
Accordingly, although there have been advances in the field, there remains a need for improved methods of protecting fuel cells from the adverse effects of voltage reversal conditions. The present invention addresses these needs and provides further related advantages.